In manufacturing lenses, and particularly lenses for use with eyeglasses, the use of plastics is often desirable due to their weight and durability. Normally in manufacturing plastic lenses, two molds in conjunction with a gasket are placed in a spaced relationship to provide a mold cavity therebetween. This mold cavity ultimately forms the shape, curvature, thickness, and configuration of the product lens. Plastic material is injected into the mold cavity and cured to harden the lens with the exterior surfaces being configured to a particular patient's prescription. In the past, to achieve the final desired surface configuration and optical quality, additional grinding and polishing of each lens may have been required. In large opthalmic facilities, it is economically feasible to employ lens grinding apparatus and technicians to grind the lenses at the site for each patient. For smaller offices, however, the capital costs and operating expenses involved in the grinding process are typically prohibitive.
This invention relates to a method for molding and curing prescription quality plastic lenses in a short period of time to produce a lens satisfactory to the patient while being economically feasible for the small office and also create economies for the entire optical industry. A feature of the invention is to limit post curing once the lens has reached the desired hardness.
One approach in molding plastic lenses is the use of a liquid monomer and a thermal initiator or catalyst in conjunction with a heat source to effect thermal curing of the lens. See Hungerford et al., U.S. Pat. No. 3,038,210, issued June 12, 1962, and Grandperret, U.S. Pat. No. 3,222,432, issued Dec. 7, 1965. Hungerford et al. and Grandperret have shown thermal curing to be a time-consuming method of producing optical quality plastic lenses. The process of thermal curing, or heat-induced polymerization of the liquid monomer, takes at least fourteen (14) to nineteen (19) hours to complete, plus any additional grinding, polishing, and finishing time required to provide a lens of optical quality.
Another approach in molding plastic lenses is the use of a liquid monomer and a photosensitive initiator or catalyst in conjunction with an ultraviolet light source to effect polymerization or curing of the lens. See Neefe, U.S. Pat. No. 4,166,088, issued Aug. 28, 1979, and Mutzhas, U.S. Pat. No. 4,298,005, issued Nov. 3, 1981. Although curing with ultraviolet light may be accomplished faster than thermal curing, it can still require up to two (2) hours or more to complete and also produces lenses with unacceptable optical aberrations caused by uneven curing and stress. Ultraviolet light curing has also been found to produce lenses that are discolored and unclear and of insufficient hardness.
The invention described herein is a new method for curing plastic lenses that overcomes many of the problems inherent in both the processes of thermal and ultraviolet light curing and the lenses produced thereby. The invention relates to the use of a lens material mixture of liquid monomer, a thermal initiator, plus a photosensitive initiator in conduction with a mold apparatus and utilization of both thermal curing and ultraviolet light curing processes, either simultaneously or separately. In this process, the liquid monomer lens material is injected into the desired ultraviolet-light-transparent mold apparatus and subjected while therein to heated fluid curing for a short period of time, less than ten (10) minutes. It is believed that this heated fluid curing acts in conjunction with the thermal initiator to form the lens material mixture into a gel which freezes the photosensitive initiator in place throughout the lens material. Furthermore, it is believed this gelled state preestablishes the optical framework needed for an optical lens free of optical distortion or imperfections. After the lens material mixture has sufficiently gelled, it is then subjected to ultraviolet light to activate the photosensitive initiator and complete the polymerization or curing process. The process of this invention has produced a lens of optical quality in less than thirty (30) minutes.
In another embodiment, the same end result can be accomplished by utilization of UV initiator only by first using a known intensity and wavelength of a UV Light source to accomplish gelling of the resin and then using a different intensity and or wavelength or both to finish the curing process.
An additional problem encountered in thermal curing, is what is referred to as a "thermal runaway," a problem created by the exothermic nature of the polymerization of the liquid monomer lens material used with a thermal initiator. An alternative embodiment of Applicant's invention employs curing the lens material in a microwave oven using a liquid cooling system to control mold temperature to avoid the problem of "thermal runaway." the mold is supported within a rotating container through which cooling liquid will pass. The cooling liquid is delivered from a tank adjacent to the microwave oven, pumped through the oven at a predetermined rate, and passed over the mold to withdraw the heat of the exothermic reaction caused by the microwave effect on the monomer and then out of the oven into a holding tank for recycling after first being cooled. In addition, the mold will be rotated to insure that the microwave effect and the fluid cooling effect occur evenly over all the surfaces of the mold. With this approach, "thermal runaway" is avoided.
Another approach is to prepare a monomer with either a thermal initiator, or a UV initiator, and/or both. After the molding material has been cured using heat, UV, or both, the lens may not be fully cured. To further harden the lens or otherwise complete the cure, the lens is withdrawn from the mold and subjected to UV rays or both.
A prepolymerization step can also be used to enhance the curing process. With this approach, the monomer is partially polymerized prior to delivery into the mold. This speeds the curing process and avoids the extensive cooling, particularly for thermally induced reactions. When utilized with the microwave technique discussed above, depending upon the percentage of thermal initiator used the cooling step may be deleted altogether. After the curing within the oven, the mold can be removed and finally cured by UV or heat, depending on the type of initiators used. When used with UV cure the thermally prepolymerized resin intervening heating can be eliminated. The prepolymerized resin can proceed directly to the UV exposure, and achieve the same satisfactory end point. However, the curing time should not be as fast as the process where initially a gel is obtained.
An additional advantage of prepolymerization is the reduction of shrinkage during the molding process. Substantial shrinkage can cause fractures and pre-releases of the molding material. By reducing this shrinkage, even by 4%, reduces these problems and increases yield.
Another approach contemplated by Applicant's invention is the use of cobalt treatment. This process takes approximately five (5) hours to complete and produces lenses with a blue coloring.
With those lenses utilizing a UV initiator, it is possible that inadvertent curing can occur. Once the curing process is completed, if the lenses are subjected to UV Light in any fashion, continued curing can occur. This can result in unacceptable brittleness in lenses, detracting from their use in the marketplace.
In the invention described herein, this post curing problem has been overcome by the use of UV inhibitors. After the curing step discussed above, the cured lens is treated with inhibitors to avoid entirely or substantially reduce the UV post curing effects.
The above is a brief discussion of certain features of Applicant's invention. Other features and advantages of the invention will be appreciated from the detailed discussion of the preferred embodiments below.